1. Technical Field
The present invention relates to a polymer, optical elements, and photoelectric device employing the same, and in particular relates to a high refractive polymers, optical elements, and optical elements employing the same.
2. Description of the Related Art
Light emitting diodes (LEDs) have high brightness, low volume, low power consumption and long life and such as, are used in a variety of display products. Recently, light emitting diodes (LEDs) have been applied widely, with high brightness LEDs heavily demanded. A total light extraction efficiency of an LED is determined by a diode chip, packaging style and encapsulation material. Currently, internal light extraction efficiency of an LED chip has reached more than 90%, but the total light extraction efficiency of an LED is only 30%. The low total light extraction efficiency of an LED is due to large refractive index differences between LED chips and conventional transparent encapsulations. Light emittance of an LED chip will pass through the conventional encapsulation and produce a total reflection. Light emitting from the LED chip is limited by the interior of LED package such that the total light extraction efficiency of the LED is reduced. Therefore, an LED encapsulation material with high refractive index is desired.
According to Snell's law, light traveling from a region having a high index of refraction to a region with a low index of refraction that is within a certain critical angle (relative to the surface normal direction) will cross to the lower index region. Light that reaches the surface beyond the critical angle will not cross but will experience total internal reflection (TIR). In the case of an LED, the UR light can continue to be reflected within the LED until it is absorbed. Because of this phenomenon, much of the light generated by conventional LEDs is not emitted, degrading its efficiency.
For example, a white light emitting diode (LED) chip has a refractivity of about 2-4, such as GaN film with a refractivity of 2.5, and GaP film with a refractivity of 3.45. Since the refractivity of a light emitting diode chip is much higher than that of conventional encapsulation material (such as epoxy resin or silicone resin with a refractivity of 1.40-1.53), the great refractivity difference between the light emitting diode chip and the encapsulation material causes a total internal reflection at an interface therebetween, resulting in a part of the emitted light being trapped within the light emitting diode chip until it is absorbed. In the white light emitting diode chip employing a blue light emitting diode (with a refractivity of 2.5) and a yellow phosphor (YAG), the white light emitting diode chip has a 30% increase in light extraction efficiency, when the refractivity of the encapsulation material is increased from 1.5 to 1.7. Therefore, the total internal reflection can be reduced by increasing the refractivity of the encapsulation material for reducing the refractivity difference between the light emitting diode chip and the encapsulation material.
In order to solve the aforementioned problems, U.S. Pat. No. 5,633,331 discloses an encapsulation material with high refractivity. The encapsulation material is prepared by blending a fluorene carbonate polymer and polysulfone.
U.S. Pat. No. 7,446,159 also discloses an encapsulation material with high refractivity. The encapsulation material is prepared from a fluorene monomer with acrylic functional groups (having a structure represented by
wherein X is —(CH2CH2O)n—, —(CH2CH2O)—CH2CH(OH)CH2O—, n is 1-5, and R is an acrylic group or methacrylic group). Since the above monomer has high viscosity, the encapsulation material exhibits poor film forming properties. Thus, it is difficult to form a film of the encapsulation material via spin coating, screen printing, or mold injection processes.
U.S. Pub. 2008/0114100A1 discloses an encapsulation material with high refractivity. The encapsulation material is prepared by blending a fluorene having acrylic functional groups with a compound having a structure represented by
However, the encapsulation material has hardness of about Shore A 90, and exhibits poor flexibility and thermal stress relaxation.